In recent years, attention has been paid to a radio train control system (CBTC system: Communication-Based Train Control System) that performs radio communication between a train and a radio base station set along a railroad track and performs operation control and speed control for the train on the basis of information transmitted by the radio communication. Compared with a conventional train operation control system by a fixed block section, the CBTC system is advantageous in terms of introduction costs and maintenance costs because a track circuit is unnecessary. Further, a flexible block section free from a fixed section can be established. Therefore, the CBTC system is also advantageous in terms of operation costs because it is possible to increase operation density of trains.
In general, in the CBTC system, an ISM (ISM: Industry Science Medical) band, which does not require a license, is often used for radio communication between the ground and a train from the viewpoint of costs. However, the ISM band is widely used in other systems such as a wireless LAN (Local Area Network) and a Bluetooth (registered trademark). The use of the other systems in trains and in buildings along a railroad line could be a significant interference source for the CBTC system.
As one of communication systems excellent in interference resistance, there is an FHSS (Frequency Hopping Spread Spectrum) system defined in, for example, IEEE (Institute of Electrical and Electronic Engineers) 802.11 (see Non Patent Literature 1). The FHSS system is a system for switching a carrier frequency in use at fixed time intervals to perform communication. Therefore, when an interference source is present in a specific frequency channel or in an environment where a frequency at which interference is present changes with time, the FHSS system avoids interference to realize stable communication by repeatedly transmitting the same data while switching a carrier frequency.
A radio communication apparatus disclosed in Patent Literature 1 further includes, in the FHSS system, a plurality of antennas and uses different hopping sequences synchronizing with one another among the antennas to thereby enable communication by using at least any one of channels and improve interference resistance while maintaining the response speed.